1. Field of the Invention
The present invention relates generally to antimicrobial agents and, more specifically, to cyclic theta defensin peptides and methods of using a theta defensin to reduce or inhibit microbial growth or survival.
2. Background Information
Infections by microorganisms, including bacteria, viruses and fungi, are a major cause of human morbidity and mortality. Although anyone can be a victim of such infection, the sick and elderly are particularly susceptible. For example, hospitalized patients frequently acquire secondary infections due to a combination of their weakened condition and the prevalence of microorganisms in a hospital setting. Such opportunistic infections result in increased suffering of the patient, increased length of hospitalization and, consequently, increased costs to the patient and the health care system. Similarly, the elderly, particularly those living in nursing homes or retirement communities, are susceptible to infections because of their close living arrangement and the impaired responsiveness of their immune systems.
Numerous drugs are available for treating infections by certain microorganisms. In particular, various bacterial infections have been amenable to treatment by antibiotics. However, the prolonged use of antibiotics since their discovery has resulted in the selection of bacteria that are relatively resistant to these drugs. Furthermore, few if any drugs are effective against microorganisms such as viruses. As a result, continuing efforts are being made to identify new and effective agents for treating infections by a variety of microorganisms.
The identification of naturally occurring compounds that act as antimicrobial agents has provided novel and effective drugs. Many organisms protect themselves by producing natural products that are toxic to other organisms. Frogs, for example, produce a class of peptides, magainins, which provide a defense mechanism for the frog against potential predators. Magainins have been purified and shown to have antimicrobial activity, thus providing a natural product useful for reducing or inhibiting microbial infections.
Natural products useful as antimicrobial agents also have been purified from mammalian organisms, including humans. For example, the defensins are a class of peptides that have been purified from mammalian neutrophils and demonstrated to have antimicrobial activity. Similarly, indolicidin is a peptide that has been isolated from bovine neutrophils and has antimicrobial activity, including activity against viruses, bacteria, fungi and protozoan parasites. Thus, naturally occurring compounds provide a source of drugs that are potentially useful for treating microbial infections.
Upon identifying naturally occurring peptides useful as antimicrobial agents, efforts began to chemically modify the peptides to obtain analogs having improved properties. Such efforts have resulted, for example, in the identification of indolicidin analogs which, when administered to an individual, have increased selectivity against the infecting microorganisms as compared to the individuals own cells. Thus, the availability of naturally occurring antimicrobial agents has provided new drugs for treating microbial infections and has provided a starting material to identify analogs of the naturally occurring molecule that have desirable characteristics.
Although such natural products and their analogs have provided new agents for treating microbial infections, it is well known that microorganisms can become resistant to drugs. Thus, a need exists to identify agents that effectively reduce or inhibit the growth or survival of microorganisms. Th e present invention satisfies this need and provides additional advantages.
The present invention relates to an isolated cyclic theta defensin peptide, which exhibits broad spectrum antimicrobial activity, and to theta defensin analogs. In general, a theta defensin or theta defensin analog has the amino acid sequence Xaa1-Xaa2-Xaa3-Xaa4-Xaa5-Xaa1-Xaa6-Xaa4-Xaa4-Xaa1-Xaa1-Xaa6-Xaa4-Xaa5-Xaa1-Xaa3-Xaa7-Xaa8, wherein Xaa1 independently is Gly, Ile, Leu, Val or Ala; Xaa2 is Phe, Trp or Tyr; Xaa3 is Cys or Trp; Xaa4 independently is Arg or Lys; Xaa5 is Cys or Trp; Xaa6 is Cys or Trp; Xaa7 is Thr or Ser; and Xaa8 is Arg or Lys. Xaa1 can be linked through a peptide bond to Xaa8. Furthermore, crosslinks can be formed between Xaa3 and Xaa3, between Xaa5 and Xaa5, and between Xaa7 and Xaa7. For example, the invention provides theta defensin having the amino acid sequence Gly-Phe-Cys-Arg-Cys-Leu-Cys-Arg-Arg-Gly-Val-Cys-Arg-Cys-Ile-Cys-Thr-Arg (SEQ ID NO:1), wherein the Gly at position 1 (Gly-1) is linked through a peptide bond to Arg-18, and wherein disulfide bonds are present between Cys-3 and Cys-16, Cys-5 and Cys-14, and Cys-7 and Cys-12.
The invention also relates to methods of using a theta defensin or an analog thereof to reduce or inhibit microbial growth or survival in an environment capable of sustaining microbial growth or survival by contacting the environment with theta defensin. As such, the invention provides methods of reducing or inhibiting microbial growth or survival on a solid surface, for example, surgical instruments, hospital surfaces, and the like.
The invention further relates to methods for reducing or inhibiting microbial growth or survival in an individual, particularly a mammal such as a human. Thus, the invention provides methods of treating an individual suffering from a pathology characterized, at least in part, by microbial infection, by administering theta defensin or an analog thereof to the individual, thereby reducing the severity of the pathologic condition.